In 1922, Niels Bohr won the Nobel Prize for Physics. He won it “for his services in the investigation of the structure of atoms and of the radiation emanating from them.” Naturally then, his Nobel Lecture, which he delivered on December 11, 1922, discussed the field of physics to which his work belonged. His lecture was titled “The structure of the atom.”
He therefore spoke about the structure of the atom and how he had been able to modify the planetary model of Ernest Rutherford to account for the hydrogen spectrum. By doing this, he had become the first to apply the quantum concept, which restricts the energy of a system to certain discrete values, to the problem of atomic and molecular structure.
He also spoke about the relationships between elements, the natural system of the elements, the X-ray spectra and atomic constitution. Before drawing to a close, he alluded to the work just done by Dirk Coster and Georg Charles von Hevesy, suggesting that their investigations might be pointing to the element with atomic number 72.
The missing element
To understand this, we would need to turn our attention to another field of work that was as important as the atomic theory. For Dimitri Mendeleev’s periodic table not only allowed for elements to be grouped in a systematic fashion, it also provided gaps to be filled in with elements as yet not discovered. One of those gaps fell for element number 72.
Even though the initial reaction towards Mendeleev’s periodic table was one of scepticism, its utility and effectiveness at what it did was soon obvious. That meant that there were several chemists soon working with this as a starting point, trying to find the missing elements. Number 72 was placed below titanium and zirconium, and therefore zirconium minerals were used to get to the new element.
It was under these circumstances that Hevesy and Coster worked on the problem of identifying the missing element. Hevesy, a Hungarian chemist, was invited by Bohr to work in Copenhagen from 1920 and Coster, a Dutch physicist, joined Bohr in 1922. Bohr too had suggested that an ore of zirconium might be a good place to start out, and they did just that.
Search in zircons
Hevesy and Coster took to Norwegian and Greenland zircons, a silicate of zirconium. On subjecting these zircons to X-ray spectroscopy and analysing it, they were able to establish the existence of element number 72 in the minerals. The chemical properties of the newly found element were similar to that of zirconium, further confirming the discovery.
Coster informed Bohr about their analysis and Bohr amended his lecture in order to include the announcement of the discovery. While both Bohr and Hevesy had liked the name danium (after Denmark) for the new element, Coster’s suggestion of hafnium (after Hafnia, new Latin name of Copenhagen, the place of discovery) prevailed at the time of publication in 1923.
